1) diffuse/base
2) dark
3) detail
4) gloss
5) glow
6) bump
7) normal ( yes, vanilla engine allows us to have both of them. Don't try it, honestly, but there are separate slots for them in the nif shader.)
8) decal 0
9) decal 1
10) decal 2
11) decal 3 (though I didn't see more then 2 decals even on meshes from mods)
2/3: I'm not sure any meshes use both the dark and detail map as the maths involved is nearly identical, so they produce the same effect.
6/7: The vanilla engine doesn't actually support either of these. MGE changes how the environment map is interpreted so if you put a normal or bump map in its slot, it'll look sort-of right. NifSkope uses the MGE convention rather than the vanilla Morrowind one, so the environment map slot is doubled up and given a different name. If you load a Nif using the MGE convention into OpenMW, it'll likely look metallic and shiny.
8-11: I think OpenMW probably bakes decals into a single texture as the shader only has a single texture slot for decals.
The OpenMW texture layout is as follows. It's not super-critical to the discussion here, but textures are assigned to the lowest numbered available slot, so if there's no diffuse map, for example, the dark map will go in slot 0 instead.
a: Diffuse. If present, sets the base colour for the pixel and its transparency. Otherwise, it starts as white.
b: Dark. Applied after the Detail map. Multiplied into the base colour.
c: Detail. Applied before the Dark map. Multiplied into the base colour, but zoomed in 2x.
d: Decal. Mixed into the base colour based on the value of its alpha channel.
e: Emissive. Added to the pixel brightness after lighting calculations.
f: Normal. Modulates surface normals to make lighting and environment reflection look better. Alpha channel is the height used for parallax, if enabled.
g: Environment. Added on to the pixel brightness after lighting calculations based on which bit of this texture is reflected.
h: Specular. Colour affects specular colour. Alpha affects shininess used for Phong highlights.
(If you're using the shadow branch, as many slots as you specify shadow maps in your settings get used, too.)
In OpenMW, as the non-MGE Morrowind Nif format doesn't have a normal map slot, these are specified outside of the Nif file.